The Redmine Editor Preview Tab Extension adds a preview tab to the Redmine text editor, similar to the editor on Github.
$ cd redmine/plugins
$ git clone https://github.com/tleish/redmine_editor_preview_tab
restart Redmine
By default, both tab and the default Redmine preview functionality is still enabled. You can disable/hide the default Redmine preview in the plugin settings.
Go to Adminstration > Plugins > Redmine Editor Preview Tab Extension
